FIG. 7 shows a circuit configuration of a resonant load power conversion device (an AC-DC conversion device) connected to a resonant load. In FIG. 7, the AC-DC conversion device 10 has a single-phase inverter whose input side is connected to a DC voltage source 11 and whose output side is connected to the resonant load 12 such as an induction heating circuit. By performing ON/OFF control of each switching element of this single-phase inverter, the AC-DC conversion device 10 outputs a rectangular wave voltage of resonance frequency to the resonant load 12 (the AC-DC conversion device 10 outputs a rectangular wave voltage to the resonant load 12 at resonance frequency).
In a case where the resonant load 12 is the induction heating circuit, the AC-DC conversion device 10 is configured as an induction heating resonant load AC-DC conversion device (an induction heating resonant type inverter).
This induction heating resonant load AC-DC conversion device is configured so that an alternating current generated by the ON/OFF control of each switching element of the single-phase inverter flows in an LC resonance circuit formed by a coil and a capacitor, an eddy current flows in a heating part (an electric conductor) by an alternating field, which is generated by the flow of the alternating current, being provided to the heating part (the electric conductor), and the LC resonance circuit is heated from an inside of the LC resonance circuit by Joule heat generated by the flow of the eddy current.
Regarding the induction heating circuit as the resonant load connected to the output side of the resonant load power conversion device (e.g. the AC-DC conversion device 10 in FIG. 7), it has been known that the higher the frequency is, the more the depth of penetration of current is decreased.
Since an electro-resistance-welded tube joint (a joint of a tube is connected by electric resistance welding for forming the tube) is performed by surface quenching (surface hardening), the resonant load AC-DC conversion device used for the induction heating is required to be able to output high frequency voltage.
On the other hand, the switching element of the resonant load AC-DC conversion device used for the induction heating has an upper limit of a drive frequency. Therefore, the resonant load AC-DC conversion device has a problem of failing to respond to a voltage frequency that is higher than the drive frequency of the switching element.
As a prior art that solves this problem, for instance, a resonant load inverter system disclosed in Patent Document 1 has been proposed. As disclosed in FIG. 3 and pars. [0007] to [0009] in this Patent Document 1, by dividing the single-phase inverter into n sections (by N-parallel connecting the single-phase inverter), the switching element can be driven at a 1/N period. As a consequence, the drive frequency of the switching element can be decreased to a frequency that is inversely proportional to the number of parallel connection with respect to a desired resonance frequency.
Further, as a modified example of the resonant load inverter system of Patent Document 1, for instance, as shown in FIG. 8, it can be conceivable that the switching elements (e.g. IGBTs) per one arm of the single-phase inverter will be configured to be connected in N parallel.
FIG. 8 shows the resonant load AC-DC conversion device, e.g. a device used for the AC-DC conversion device 10 shown in FIG. 7. The resonant load AC-DC conversion device has a DC link voltage input section Vdc, a rectangular wave voltage output section Vout, and a single-phase inverter in which N-parallel switching elements per one arm (here, 3 parallel) (U11, U21, U31, V11, V21, V31, X11, X21, C31, and Y11, Y21, Y31) are connected.
As shown in FIG. 8, by increasing the number N of the parallel-connected switching elements per one arm, as same as the resonant load inverter system disclosed in Patent Document 1, it is possible to decrease the switching frequency per one switching element.
Each switching element in FIG. 8 is ON/OFF-controlled along a gate command signal generating pattern shown in FIG. 9.
The gate command signal generating pattern of FIG. 9 is formed from                a clock with ON and OFF of an output voltage command (Vout_ref) of the single-phase inverter being a trigger,        a switching element U11 and Y11 gate command signal U11_gate/Y11_gate with 6 clocks being one period (one cycle) and with an ON signal being outputted at 1 clock and an OFF signal being outputted at 5 clocks,        a switching element X11 and V11 gate command signal X11_gate/V11_gate that is delayed by 1 clock with respect to the gate command signal U11_gate/Y11_gate and has the same ON and OFF periods as ON and OFF periods of the gate command signal U11_gate/Y11_gate,        a switching element U21 and Y21 gate command signal U21_gate/Y21_gate that is delayed by 1 clock with respect to the gate command signal X11_gate/V11_gate and has the same ON and OFF periods as ON and OFF periods of the gate command signal X11_gate/V11_gate,        a switching element X21 and V21 gate command signal X21_gate/V21_gate that is delayed by 1 clock with respect to the gate command signal U21_gate/Y21_gate and has the same ON and OFF periods as ON and OFF periods of the gate command signal U21_gate/Y21_gate,        a switching element U31 and Y31 gate command signal U31_gate/Y31_gate that is delayed by 1 clock with respect to the gate command signal X21_gate/V21_gate and has the same ON and OFF periods as ON and OFF periods of the gate command signal X21_gate/V21_gate, and        a switching element X31 and V31 gate command signal X31_gate/V31_gate that is delayed by 1 clock with respect to the gate command signal U31_gate/Y31_gate and has the same ON and OFF periods as ON and OFF periods of the gate command signal U31_gate/Y31_gate.        
Each switching element in FIG. 8 is ON/OFF-controlled by each generated gate command signal of U11_gate/Y11_gate . . . X31_gate/V31_gate with patterns (1) to (6) shown in FIGS. 10A to 10F being repeated.
A relationship of an output current when driving each switching element in FIG. 8 by the gate command signal generating pattern shown in FIG. 9 is illustrated in FIGS. 10A to 10F.
FIGS. 10A to 10F correspond to the patterns (1) to (6) of FIG. 9. In FIGS. 10A to 10F, the switching element that is ON-controlled by the ON signal of the gate command is indicated by “ON”, and a route of an output current Iout passing through the ON-controlled switching element and a load is shown by an arrow.
According to FIGS. 9 and 10A to 10F, by switching each switching element sequentially by the patterns (1) to (6), it can be understood that a frequency index of the switching frequency (the drive frequency) per one switching element is 1/3 (1/N).